Methane dehydroaromatization by Mo/HZSM-5: mono- or bifunctional catalysis?

The active site requirements for methane dehydroaromatization by Mo/HZSM-5 were investigated by using physical mixtures of Mo-bearing supports (HZSM-5, SiO2, -Al2O3 and activated carbon) and acidic HZSM-5. The use of two different sizes of shaped particles allowed separating these two catalyst components after activation and reaction. In this way, we could demonstrate that migration of volatile Mo-oxides onto the zeolite is at the origin of the observed catalytic synergy in methane dehydroaromatization for the physical mixtures. The propensity of Mo migration depends on the activation method and the Mo-support interaction. Migration is most pronounced from Mo/SiO2. Prolonged exposure of HZSM-5 zeolite to Mo-oxide vapors results in partial destruction of the zeolite framework. Mo-carbide dispersed on non-zeolitic supports afforded predominantly coke with only very small amounts of benzene. The main function of the zeolite is to provide a shape-selective environment for the conversion of methane to benzene. The main role of the Brønsted acid sites is to promote the dispersion of the Mo-oxide precursor in the micropores of HZSM-5. We show that Mo-carbide species dispersed within non-acidic Silicalite-1 also show reasonable activity in methane dehydroaromatization.